Vulcanization of rubber

ABSTRACT

WHEREIN R REPRESENTS A RUBBER MOLECULE, AR IS AN AROMATIC GROUP, Y IS AN OXYGEN ATOM OR A SUBSTITUTED OR UNSUBSTITUTED-NH-GROUP, AND Q IS AN ORGANIC DIFUNCTIONAL GROUP.   R-NH-AR-Y-CO-NH-Q-NHCO-Y-AR-NH-R   THE PRESENT INVENTION RELATES TO A NOVEL METHOD OF CROSSLINKING NATURAL OR SYNTHETIC RUBBERS WHICH COMPRISES REACTING RUBBER WITH A NITROSOPHENOL OR NITROSAMINE AND REACTNG PENDENT AMNO OR HYDROXYL GROUPS IN THE RESULTING PRODUCT WITH A DI- OR POLY-ISOCYANTE, SO AS TO CROSSLINK THE RUBBER. NOVEL SYNTHETIC RUBBER VULCANIZATES ARE ALSO PROVIDED WHEREIN THE CROSSLINKS OF SAID VULCANIZATES HAVE THE GENERAL FORNULA:

United States Patent US. Cl. 26077.5 CR Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a novel method of crosslinking natural or synthetic rubbers which comprises reacting rubber with a nitrosophenol or nitrosoamine and reacting pendent amino or hydroxyl groups in the resulting product with a dior poly-isocyante, so as to cross link the rubber. Novel synthetic rubber vulcanizates are also provided wherein the crosslinks of said vulcanizates have the general formula:

wherein R represents a rubber molecule, Ar is an aromatic group, Y is an oxygen atom or a substituted or unsubstituted NH group, and Q is an organic difunctional group.

This invention provides a system for cross-linking rubher which makes use of nitrosoanilines or nitrosophenols. The principle of the system is illustrated by the following schematic reaction diagram in which two rubber molecules, R, react with the nitroso groups of two nitrosophenol molecules, and the cross-link is completed by reaction of the pendent hydroxyl groups with a di-isocyanate.

3,645,980 Patented Feb. 29, 1972 The invention provides a method of cross-linking a natural or synthetic rubber, which method comprises reacting the rubber with an aromatic nitroso-compound having the formula XArNO, where X is a hydroxyl or a primary or secondary amino group and Ar is an aromatic group, and reacting the pendent amino or hydroxyl groups in the resulting product with a polyfunctional linking agent so as to cross-link the rubber.

The nitroso-compound is one having a nitroso group, attached to a carbon atom of an aromatic ring, which is capable of adding to an unsaturated rubber molecule, and also having at least one hydroxyl or amine group capable of reacting with the linking compound. One hydrogen atom of the amine group may be replaced, provided that the reactivity of the amine group towards the linking compound is not thereby nullified. Thus, We may use ortho-, metaor para-nitrosoaniline or ortho-, metaor para-nitrosophenol. We may also use analogues of these compounds in which the aromatic ring carries one or more inert substituents, such as alkyl or aryl groups, or forms part of a fused aromatic ring system, provided that such substituents are not so large and so positioned as to prevent the functional groups of the nitrosophenol or nitrosoaniline from reacting with the linking compound. The preferred nitroso compounds are para-nitrosoaniline (PNA) and para-nitrosophenol (PNP). PNA is more reactive than PNP and less prone to thermal decomposition but vulcanizing temperatures may be inconveniently low. PNP is less eflicient than PNA, but cross-linking temperatures are of the order normally used for sulphur curing.

As a linking compound, we may use dior poly-isocyanates including aromatic compounds, for example, toluene-2,4-di-isocyanate, and aliphatic compounds, for example, that sold by E. I. Du Pont de Nemours and Co. Inc. under the trademark Hylene W, believed to be 4,4- di-isocyanato-dicyclohexylmethane. It is Within the scope of the invention to use trior polyfunctional compounds as the linking compounds.

When the linking compound is a di- (or poly-)iso- The system of this invention is particularly suitable for use with natural rubber. It is, however, also suitable for use with all natural and synthetic rubbers containing unsaturated carbon-carbon linkages, or other groups capable of reaction with aromatic nitroso groups, in appreciable amounts. The system is not suitable for use with those rubbers which contain very low amounts of unsaturation for vulcanization purposes, for example, ethylene-propylene terpolymers and butyl rubber.

There have been previous proposals for cross-linking butyl rubber by the use of di-nitroso compounds. In the system of this invention, it is believed that the unsaturated rubber molecules react with mono-nitroso compounds to provide reactive pendent groups and that the cross-link issubsequently completed by means of a linking compound.

cyanate, the rubber vulcanizates may be represented as having cross-links with the following general formula:

may be desirable to add to the rubber mix a drying agent of such a nature and in such an amount as to remove the water by reacting chemically with it. Calcium oxide is an example of such a drying agent, and a suspension of calcium oxide in oil sold under the trademark Caloxol is particularly suitable. The use of a drying agent is not a necessity; but it is convenient and cheap and has no attendant disadvantages.

One advantage of the system of this invention lies in the fact that the properties of the vulcanizate may be readily altered by altering the length of the crosslinking chain. Long cross-linking chains can readily be achieved by using higher molecular weight linking compounds, for example, those di-isocyanates sold by E. I. Du Pont de Nemours and Co. Inc. under the trademark Adiprene.

The nitroso compound, or the linking compound, or both, may be formed in situ in the rubber mix, rather than being added per se. The nitroso compound and the linking compound, or their respective precursors, may be added to the rubber mix either together or separately in either order, as is more fully described below. Thorough mixing with the rubber is necessary, and this may conveniently be achieved by milling, for example, in an open mill or an internal mixer. The cross-linking system of this invention may be used either alone, or in conjunction with other systems, e.g., using sulphur, in order to vulcanize the rubber, provided that the other vulcanizing ingredients do not interfere with the nitroso-amine or phenol, or with the linking compound. The nitroso compound and the linking compound may be added to the rubber before, together with, or after such other fillers, additives, or other compounding ingredients as may be determined for the subsequent application of the vulcanizate by the usual practice of the art.

The amount of the nitroso compound and linking compound added to the rubber will depend upon the degree of cure required and can quite readily be determined by methods known in the art. Thus the proportion of aromatic nitroso compound may be from 0.5 to 10, preferably 1 to 5, parts by weight per hundred parts of dry rubber, and the proportion of linking compound may be from 0.5 to 15, preferably 1 to 10, parts by weight per hundred parts of dry rubber. The nitroso compound and the linking compound may be used in equivalent amounts, or either may be used in excess. However, when isocyanates are used as linking compounds, better crosslinking efliciencies may be obtained when excess isocyanate is employed, as the excess may be able to cross-link the cross-links.

A most important advantage of the present invention is that vulcanizates prepared according to it are virtually reversion-resistant. Conditions of cure are therefore not critical, provided always that the temperature of cure is sufiicient to form in situ any of the nitroso compound and the linking compound which may not have been added to the rubber as such. Curing conditions may, for example, range from 100 C. to 200 C., for from 6 hours to 10 seconds, e.g., from 1 hour to 10 minutes.

We have further discovered that the cross-linking efficiency of the system may be significantly improved by the addition of certain metal salts of thiols. Examples of such salts include zinc, cadmium and stannous dithiocarbamates, particularly dialkyldithiocarbamates, dithiophosphates, particularly dialkyldithiophosphates, and mercaptobenzothiazoles. It is to be expected that salts of other thiols and thio-acids, in which the (divalent) metal atom is directly bonded to sulphur, will also be effective. Among the salts, the preferred ones are zinc dimethyl-, diethyland di-n-butyl-dithiocarbamates on account of their ready availability.

The amount of the metal thio salt used is not critical, and may suitably be from 0.5 to 10, preferably 1 to 6, parts by weight per hundred parts of dry rubber. Generally, 2 parts of the salt are sulficient to give the desired efficiency increase, and there is little to be gained by going above this figure.

According to one aspect of the present invention, the linking compound may be added to a pre-reaction product of the rubber with the nitroso compound. The reaction between natural rubber and PNA or PNP may be effected by heating the two together at a temperature of from C. to 250 C., preferably from C. to 200 C., for suitable reaction times, higher temperatures requiring shorter reaction times. When an internal mixer is used, reaction between the rubber and the nitroso compound may conveniently be effected during mixing. Alternatively the reaction may be effected in latex or in wet coagulum according to the methods described in United States patent application No. 856,828, filed Sept. 10, 1969 of Douglas Bernard et al.

This process suffers, however, from the disadvantage that the linking compound may react too readily with the pendent amine or hydroxyl groups. Thus, when using PNP as the nitroso compound and toluene-2,4-diisocyanate as the linking compound, we have found it difiicult to avoid premature vulcanization during milling of the link ing compound into the pre-reacted rubber.

It may be possible to mitigate this problem either by keeping the temperature down during the mixing of the linking compound with the pre-reacted rubber (e.g., by using a cooled open mill), or by chemically modifying either the PNP or the di-isocyanate or both so as to reduce the reactivity of the pendent groups towards the linking compound. Nevertheless we prefer to avoid the problem by adding the di-isocyanate in the form of a precursor which decomposes in situ at an elevated temperature to give the linking compound itself. One such compound is the his phenol adduct of methylene-bis-(4-phenyleneisocyanate), which is commercially available from EXI. Du Pont de Nemours and Co. Inc. under the trademark Hylene MP. When heated to C. or above, this compound dissociates to give methylene-bis- (4-phenyleneisocyanate). Thus we may mill Hylene MP into a mix containing a pre-reaction product of the rubber with PNP, taking care that the temperature does not reach 150 C., and subsequently cure for the required length of time at a temperature of at least 150 C., for example, 30 minutes at 180 C.

According to another and more preferred aspect of the invention, the nitroso compound and the linking compound or a precursor thereof may be milled into the rubber mix together. Under these conditions we find that the premature vulcanization mentioned above is not a problem when PNP or PNA is added to the rubber at the same time as a di-isocyanate. I

It is, however, necessary to take steric factors into account when selecting a di-isocyanate. Thus we have found that the cross-linking efiiciency obtained when using PNP and Hylene W is much greater than that obtained when using PNP and the more sterically hindered toluene- 2,4-di-isocyanate. Better elficiencies still can be obtained by the use of PNA with Hylene W, although the rubber mix cures rapidly at temperatures as low as 120 C. As stated above, the efiiciency may be further improved by the addition of a metal thiol salt.

According to another, particularly preferred, aspect of the invention, the pre-reaction product of a nitrosophenol with a dior poly-isocyanate is added to the rubber, and the resulting mix heated to cross-link the rubber. It is thought that the pre-reaction product is formed by a reaction between the di-isocyanate and the nitrosophenol in its oxime form which may be exemplified where R is -(cyclo-C H (cyclo-C H This is believed to be the formula of Diurethane E, used in Examples 37 to 62 below. The diurethane thus formed is believed to subsequently decompose at an elevated temperature (in the same way as Hylene MP) to give the nitrosophenol and the di-isocyanate which then vulcanize the rubber. We have found that 140 C. to 180 C., dependent on structure, is normally suflicient to decompose the diurethane, and that cure may be conveniently eifected by heating the mix for 30 minutes at a temperature in this range.

The use of a pre-reaction product is expected to release the two cross-linking reagents in the rubber mix in stoichiometric amounts. However we find that the use of additional di-isocyanate in conjuncion with the pre-reaction products leads to an increase in cross-linking efficiency. The proportion of diurethane added to the rubber Will depend on the degree of cure required, and can quite readily be determined by methods known in the art. Proportions will generally be within the range 1 to preferably from 2 to 10, parts by weight per hundred parts of dry rubber. When an excess of di-isocyanate is used, the proportion will preferably be up to 6 parts by weight per hundred parts of dry rubber. As stated above, the excess di-isocyanate is believed to increase curing efficiency by cross-linking the cross-links. When a particular degree of cure is required, it will generally be possible to use either a large amount of diurethane and a small excess of di-isocyanate, or a smaller amount of diurethane and a larger excess of di-isocyanate. For example, in natural rubber gum vulanizates, approximately the same degree of cure may be achieved using either 4 parts of diurethane with 3 parts excess di-isocyanate, or 6 parts of diurethane with 2 parts excess of di-isocyanate, or 8 parts of diurethane with 1 part excess of di-isocyanate.

It is an important advantage of the use, according to this invention, of diurethanepre-reaction products that rubber vulcanizates prepared therefrom are generally nonstaining.

This invention includes within its scope the various processes for cross-linking unsaturated natural or synthetic rubber which are described above and in the examples which follow, and also includes the rubber vulcanizates so produced.

The following examples illustrate the invention. In the examples the following abbreviations are used:

phr.Parts per hundred parts by weight of dry rubber.

RSS 1Ribbed smoked sheets Grade 1.

HAF blackHigh abrasion furnace black.

SRF blackSemi-reinforcing furnace black.

Caloxol C31Calcium oxide suspension in oil (trademark).

Details and results of Examples 1 to 25 are tabulated in Table 1 below.

EXAMPLES 1 TO 4 Caloxol C31, PNP, and Hylene MP weer added to RSS1 in the normal manner on a cold, open mill. The mix was then press-cured, and the relaxed modulus of the vulcanizate at extension determined. It had previously been determined that the conditions of cure chosen were appropriate to this mix. The addition of the Caloxol to the mix was found to be necessary to eliminate porosity in the vulcanizate.

EXAMPLES 5 AND 6 Caloxol C31, PNP, and TDI were added to RSS1 in the normal manner on a cold, open mill. The efficiency with respect to the di-isocyanate concentration is rather lower than that of Examples 1 to 4.

EXAMPLES 7 TO 10 Caloxol C31, PNP, and Hylene W were added to RSS1 in the normal manner on a cold, open mill. A higher vulcanizing efficiency is obtained than in Examples 5 and 6 and this is believed to be due to the fact that Hylene W is less sterically hindered than TDI.

EXAMPLES 11 TO 18 PNA and Hylene W were added to RSS1 in the normal manner on a cold, open mill. In Examples 11 to 14, 17 and 18, Caloxol C31 was also added with the above reagents. Again, the vulcanizing conditions chosen had previously been shown to be appropriate to these mixes.

The results show a distinct improvement in vulcanization efficiency over that obtained using PNP. It was found that the Caloxol C31 could be omitted without rendering the vulcanizate porous. However, as may be seen by a comparison of Examples 13 and 14 with Examples 15 and 16, omission of the drying agent results in a substantial drop in modulus, presumably because part of the di-isocyanate reacts with the water present.

EXAMPLES 19 TO 22 Caloxol C31, PNA, and Hylene W were added in the normal manner on a cold, open mill to a previously prepared mix of RSS1 and HAF black.

EXAMPLES 23 TO 25 Caloxol C31, PNP and Hylene W were added to RSS1 in the normal manner on a cold, open mill. In Examples 24 and 25 proportions of ZDMC were also included with a substantial improvement in the cross-linking efliciency of the system.

TABLE 1 Mix No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 10 20 21 22 23 24 25 Composition (phr.):

RSS1 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 HAF black- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 50 50 50 50 0 0 0 5 2. 5 4 4 5 5 2. 5 2. 5 5 5 0 0 0 0 0 0 0 0 0 0 0 0 5 5 5 PNA 0 0 0 0 0 0 0 0 0 0 1 1 2 2 2 2 3 3 1 1 3 3 0 0 0 Caloxol (131..-. 5 5 5 5 5 5 5 5 5 5 5 5 5 5 0 0 5 5 5 5 5 5 5 5 5 TDI 0 0 0 0 1 5 O 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Hylene MP 7. 5 10 7 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Hylene W 0 0 0 0 0 0 1.3 2. 6 2. 6 9. 1 1 2. 5 1 4 1 4 2. 5 6 1 2 1 6 6. 5 6. 5 6. 5 ZDMC. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 12 Cure time (1m 30 30 30 30 30 30 30 30 30 30 20 20 20 20 20 20 20 20 20 20 20 20 40 40 40 Temp. C.). 180 180 180 180 180 180 180 180 180 180 120 120 120 120 120 120 120 120 120 120 120 140 140 MRlOO (kg/0111. 4. 7 5. 6 5. 1 6.0 3. 8 5. 3 3. 4 4. 2 5.0 8.0 5. 2 5. 8 6. 3 8.7 4. 3 6. 4 8.0 11. 4 15. 1 17. 7 24.0 40. 5 6. 9 11. 5 11. O

PNPPara-nitrosophenol.

PNA-Para-nitrosoaniline.

Hylene W--An aliphatic di-isocyanate of NCO content 31.8% by Weight (trademark).

Hylene MP-The bis-phenol adduct of methylene-bis-(4- 7O phenylene-isocyanate) (trademark). TDI-Toluene-2,4-di-isocyanate. TMDI2,2,4-trimethyl-hexamethylene-1,6-di-isocyanate. MRlOO-Relaxed modulus at 100% extension (BS1673 Pt. 4. Section 4.62 (1953)). ZDMCZinc dimethyldithiocarbamate.

EXAMPLES 26 TO 33 Samples of Diurethane A, B, C or D were added with Caloxol C31 to RSS1 in the normal manner on a cold, open mill. In Examples 28 to 33, additional isocyanate 8 60 to 62). The relaxed modulus at 100 extension of the vulcanizates is reported in Table 5.

TABLE 5 was included (not necessarily, it should be noted, the di- E I 58 5g 60 61 62 isocyanate from which the diurethane had been prepared). 5 e

In Examples 29, 31 and 33, ZDMC was also added. The com s ipw (pm-) 100 100 100 100 mixes were press-cured under appropriate conditions, and SEE 5155 7 7 50 50 50 O 50 th R l e vulcanizates ar iven in Table 2. Caloxol C314- 5 5 5 5 5 e M 100 va ues of th e g gflggi E w 6 4 1 0 0 z 2 2 2 10 a ta .0... 2 t a t a t 7 l e To Illa NO 25 27 28 29 31 MR100 (kg/cm!) 9.0 23.3 20.3 21,6 20.8

Composition (phr.):

RSS1 g g g g g g g g The vulcanizates prepared according to the previous 4 10 0 0 0 o 0 0 examples generally have excellent ageing properties. This g g 8 3 2 g g is demonstrated in Table 6 below in respect of the V111- 0 0 g 1 g g 3.3.; a. canizates of Examples 46, 51, 53 and 60. 0 0 0 0 0 0 2.4 2.4 0 0 TABLE 6 0 0 0 0 0 0 2. 0 2. 0 0 0 0 2 0 2 0 2 Example 45 51 53 so 30 a0 30 30 30 30 30 30 Temperature 180 180 150 150 150 150 150 150 Propert es: a MR100 (k g) 3.4 4.7 3.4 4.9 3.2 3.8 3.4 5.2 Initial tensile strength (kg./cn 1. 180 201) 212 108 Percent tensile strength retained after air oven ageing at 100 (3.: EXAMPLES 34 TO 36 g zfg g or ay s. 9 Caloxol C31, PNA, and Hylene W were added to three 122},fi fifiggggggi t gfggg2 560 595 760 380 synthetic rubbers in the normal manner on a cold, open altrer airdoven agein at 100 0.:

mill. The synthetic rubbers used were a stereoregular f 2.? t3 2% polybutadiene (Phillips Cis-4), a styrene-butadiene cog duh s l ggfla d it 10.0 8.5 7.0 29.0

polymer (Intel 1500) and a polychloroprene (Neoprene 23 38 5 C: 3 er agemg WRT). The data given in Table 3 below that these three For ays 110 85 77 102 t For 14 day 115 90 54 103 rubbers were vulcanized as readily as natural rubber using System EXAMPLES 53 TO 71 TABLE 3 The effectiveness of ZDMC in increasing cross-link 54 efficienc has been demonstrated above. Table 7 below Composition (phr.): shows that other zinc, cadmium and stannous salts of 33- 8 g 8 other thiols have a similar effect. The compounds reported Neo rene'wlif: 0 0 100 were added in the usual way on a cold open mill to RSS1, 35 33 g g 3 together with 5 phr. of Caloxol C31, 4 phr. of Diurethane H leriWjI- 4 2.5 3 E and 3 phr. of Hylene W. Proportions of the salts were $213 33 5% 33 2g chosen to be equivalent to 2 phr. of ZDMC. The mixes MR100(kg,/cm. 9.1 5.7 4.3 were vulcanized for 60 minutes at 140 C. The experiments were performed in two batches, with the control EXAMPLES 37 o 5 MR100 figure being, as expected, rather different in the two cases. PNP was caused to react with Hylene W in toluene solu- TABLE 7 tion by heating for 10 minutes at 100 C. to give a prod uct designated Diurethane E. (MP. 158 C.). EX m 16 Addy irnioo Natural rubber gum vulcanizate formulations were prea p 1 pared by mixing together 100 parts of RSS1, 5 phr. of o 2 3 5 as Caloxol C31 and various proportions of Diurethane E, Zinc ggi i g 3g g g 51g Hylene W and ZDMC in the normal manner at about 70 65 i dibutyldithiocarbmnawu 8.1

. 66.. Zine dtnonyldithiocarbamate 7. 0

C. on a cold, open mill. The mixes were cured under ap- Cadmmm dpisopmpylgithiocarbamate 9 propriate conditions (30 minutes at 140 C. for Examples 63 Cadmmm y ar amat 6.4

. 0 Control. None 3.5

37 to 41, and 60 minutes at 140 C. for Examples 42 to Zinc mercaptobemmhiazole 57). The relaxed modulus at 100% extension of the Z Cadmlum {11:1svpropylqlthlophflsphate-- 71 Stannous d1-isopropyldithiophosphate.. 4. 9 vulcanizates 18 reported in Table 4.

TABLE 4 Example 37 3s 39 40 41 42 43 44 45 46 47 4s 49 50 51 52 53 54 55 56 57 100 100 100 100 100 100 10g 10g 10g 10g 10g 102 in? 10g 100 10g 10g 100 100 100 EXAMPLES 58 TO 62 We claim:

Further samples of Diurethane B were used to prepare natural rubber filled vulcanizate compositions. The Diurethane E was mixed in the usual way with RSS1, SRF black, Caloxol C31, and in certain cases with Hylene W, ZDMC and a compounding oil (Petrofina 2069), at about 70 C. on a cold, open mill. The mixes were cured under appropriate conditions (30 minutes at 140 C. for Examples 58 and 59, and minutes at 140 C. for Examples 1. A method of cross-linking a natural or synthetic rubber containing unsaturated carbon-carbon linkages, which method comprises:

reacting the rubber with from 0.5 to 10 parts by weight per parts of dry rubber of an aromatic nitroso compound having the formula XArNO, where- X is a hydroxyl or a primary or secondary amino group, and Ar is an aromatic group;

and reacting the pendent hydroxyl or amino group in the resulting product with from 0.5 to 15 parts by weight per 100 parts of dry rubber of a polyfunctional linking compound selected from diand polyisocyanates, so as to crosslink the rubber.

2. A method as claimed in claim 1, wherein the aromatic nitroso compound is para-nitrosophenol or paranitrosoaniline.

3. A method as claimed in claim 1, wherein the polyfunctional linking compound is an organic di-isocyanate.

4. A method as claimed in claim 1, wherein from 0.5 to parts by weight per 100 parts of dry rubber of a zinc, cadmium or stannous dithiocarbamate, dithiophosphate or mercaptobenzothiazole are included in the rubber mix.

5. A method as claimed in claim 1, wherein the aromatic nitroso compound is a nitrosophenol, which with the dior poly-isocyanate is included in the rubber mix in the form a dior poly-urethane pre-reaction product in an amount of from 1 to parts by weight per 100 parts of dry rubber.

6. A method as claimed in claim 5, wherein the diurethane prereaction product is the reaction product of paranitrosophenol in its oxime form with 4,4-di-isocyanatedicyclohexylmethaue.

7. A method as claimed in claim 3, wherein calcium oxide is included in the rubber mix in an amount to remove any water present.

8. A method of cross-linking natural rubber, which method comprises the steps of:

forming a pre-reaction product by reacting para-nitrosophenol in its oxime form with an organic di-isocyanate to give a diurethane;

milling together on a cold open mill the rubber, from 2 to 10 parts by weight per 100 parts of dry rubber of the diurethane pre-reaction product, up to 6 parts by weight per parts of dry rubber of an organic di-isocyanate, from 1 to 6 parts by weight per 100 parts of dry rubber of zinc dimethyldithiocarbate, and sufficient calcium oxide to react with any water present;

and curing the rubber mix by heating it at a temperature of from about C. to about C.

9. A method of cross-linking a natural or synthetic rubber containing unsaturated carbon-carbon linkages, which method comprises adding to the rubber from 1 to 15 parts by weight per 100 parts of dry rubber of a prereaction product of a nitrosophenol, having the formula HOArNO where Ar is an aromatic group, with a dior poly-isocyanate, and heating the mixture to a temperature and for a time to cross-link the rubber.

10. A vulcanizate of natural or synthetic rubber containing unsaturated carbon-carbon linkages characterized in that the cross-links have the general formula NH-CO-YArNH-R wherein R represents a rubber molecule, Ar is an aromatic group, Y is an oxygen atom or a substituted or unsubstituted NH group, and Q is an organic difunctional group.

References Cited UNITED STATES PATENTS 2,381,063 8/1945 Kung 260-83 DONALD E. CZAIA, Primary Examiner M. J. WELSH, Assistant Examiner US. Cl. X.R. 

